U.S. patent number 5,414,548 [Application Number 08/127,879] was granted by the patent office on 1995-05-09 for arrayed-wave guide grating multi/demultiplexer with loop-back optical paths.
This patent grant is currently assigned to Nippon Telegraph and Telephone Corporation. Invention is credited to Kyo Inoue, Masao Kawachi, Yoshiaki Tachikawa, Hiroshi Takahashi.
United States Patent |
5,414,548 |
Tachikawa , et al. |
May 9, 1995 |
Arrayed-wave guide grating multi/demultiplexer with loop-back
optical paths
Abstract
An optical device is presented which is useful for optical
signal transmission and switching systems by multiplexing and
demultiplexing optical signals in looped optical paths, consisting
of a plurality of individual loop-back optical paths. The device is
essentially a multi/demultiplexer having an arrayed waveguide
grating disposed between a plurality of input sections and output
sections which are joined by the plurality of individual loop-back
optical paths. Because the modulated signals are looped back into
the same optical paths using the same devices, problems of
mismatching performance introduced by using different optical
devices are avoided. The device processes individual optical
signals of different wavelengths, minimizes splitting losses, and
reduces noise components by producing narrow bandpass signals of
high signal to noise ratio. Optical signal splitting and insertion,
delay line memory and delay equalization circuits can all be
handled by the same circuit configuration. The device is simple in
construction, reliable in performance and economical in
production.
Inventors: |
Tachikawa; Yoshiaki (Mito,
JP), Kawachi; Masao (Mito, JP), Takahashi;
Hiroshi (Yokosuka, JP), Inoue; Kyo (Yokohama,
JP) |
Assignee: |
Nippon Telegraph and Telephone
Corporation (Tokyo, JP)
|
Family
ID: |
26461174 |
Appl.
No.: |
08/127,879 |
Filed: |
September 28, 1993 |
Foreign Application Priority Data
|
|
|
|
|
Sep 29, 1992 [JP] |
|
|
4-260222 |
May 26, 1993 [JP] |
|
|
5-124488 |
|
Current U.S.
Class: |
398/87; 385/14;
385/15; 385/24; 385/37; 398/79 |
Current CPC
Class: |
G02B
6/12014 (20130101); G02B 6/12016 (20130101); G02B
6/12019 (20130101); G02B 6/12021 (20130101); G02B
6/278 (20130101); G02B 6/2861 (20130101); G02B
6/29361 (20130101); G02B 6/2938 (20130101); G02B
6/29395 (20130101); G02B 6/4215 (20130101); H01S
5/026 (20130101); H01S 5/0268 (20130101); H01S
5/1071 (20130101); H04B 2210/258 (20130101) |
Current International
Class: |
G02B
6/28 (20060101); G02B 6/34 (20060101); H01S
5/00 (20060101); H01S 5/026 (20060101); H01S
5/10 (20060101); G02B 006/26 (); H04J 014/00 () |
Field of
Search: |
;359/166,173,124,127,130,115,116 ;385/24,14,15,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Patent Abstracts of Japan, vol. 13, No. 200 (P-869), May 12, 1989
& JP-A-01 020 506..
|
Primary Examiner: Williams; Hezron E.
Assistant Examiner: Moller; Richard A.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner
Claims
What is claimed is:
1. An optical multiplexing/demultiplexing device, having loop-back
paths constituted by a plurality of optical path means, for
performing wavelength multiplexing and demultiplexing of an optical
signal consisting of a plurality of wavelengths by separating said
optical signal into individual wavelengths and propagating each
wavelength separately in each of said optical path means, said
device comprising:
a. an arrayed waveguide grating multi/demultiplexer device,
comprising an arrayed waveguide grating; a plurality of input
sections for receiving said optical signals; a plurality of output
sections for outputting optical signals; a slab waveguide for
distributing or coupling wavelength signals optically disposed
between said plurality of input sections and said waveguide
grating, and another slab waveguide for distributing or coupling
wavelength signal optically disposed between said plurality of
output sections and said arrayed waveguide grating; and
b. a plurality of loop-back optical path means for propagating and
looping optical signals comprising said plurality of optical path
means and having signal processing means disposed on selected ones
of the optical path means;
wherein said arrayed waveguide grating, a plurality of input
sections, a plurality of output sections, and said slab waveguides
are integrally fabricated on a common substrate base functioning as
the arrayed waveguide grating multi/demultiplexer device, and said
loop-back optical path means are optically connected with said
plurality of input sections and with said plurality of output
sections to propagate a wavelength signal from said plurality of
output sections into at least one input section of said plurality
of corresponding input sections to loop said wavelength signal at
least once in one optical path means of said loop-back paths,
thereby performing multiplexing and demultiplexing and signal time
delay operations using said array waveguide grating
multiplexer-demultiplexer device having loop-back paths.
2. The optical multiplexing/demultiplexing device as claimed in
claim 1, wherein each of said plurality of loop-back optical paths
means is provided with a signal processing means.
3. The optical multiplexing/demultiplexing device as claimed in
claim 1, wherein a slab waveguide is disposed between said
plurality of input sections and said plurality of output
sections.
4. The optical multiplexing/demultiplexing device as claimed in
claim 1, wherein said optical signals include a pilot optical
signal which is not looped back into said loop-back optical paths
means, and is inputted into said multi/demultiplexer having arrayed
grating and is outputted from said output section.
5. The optical multiplexing/demultiplexing device as claimed in
claim 1 wherein each of said plurality of loop-back optical paths
means has a length proportional to the wavelength of an optical
signal being propagated in an individual one of said plurality of
loop-back optical paths means.
6. The optical multiplexing/demultiplexing device as claimed in
claim 1, wherein each of said plurality of loop-back optical paths
means has a length inversely proportional to the wavelength of an
optical signal being propagated in an individual one of said
plurality of loop-back optical paths means.
7. The optical multiplexing/demultiplexing device as claimed in
claim 1, wherein at least one input section of said plurality of
input section includes a light source selected from a group
consisting of a wavelength-tunable light source, a multi-wavelength
light source and a fixed-wavelength light source.
8. The optical multiplexing/demultiplexing device as claimed in
claim 1, wherein at least one output section of said plurality of
output section includes a photodetector means.
9. The optical multiplexing/demultiplexing device as claimed in
claim 7, further comprising an optical modulator between each input
section of said plurality of input sections and said
wavelength-tunable light source.
10. The optical multiplexing/demultiplexing device as claimed in
claim 7, further comprising a polarization compensator between one
input section of said input sections and said wavelength-tunable
light source.
11. The optical multiplexing/demultiplexing device as claimed in
claim 1, wherein said optical device and said loop-back optical
paths means are fabricated integrally.
12. The optical multiplexing/demultiplexing device as claimed in
claim 1, wherein said optical device and said loop-back optical
paths means are fabricated integrally on a common substrate
base.
13. The optical multiplexing/demultiplexing device as claimed in
claim 12, wherein each of said plurality of loop-back optical paths
means includes signal processing means for processing an optical
signal being propagated in said loop-back optical paths means.
14. The optical multiplexing/demultiplexing device as claimed in
claim 13, wherein at least one of said signal processing means is
selected from a group consisting of an optical amplifier, an
optical switch means, an optical filter means, photodetector and
light source means, and optical coupler means.
15. The optical multiplexing/demultiplexing device as claimed in
claim 12, wherein each of said loop-back optical paths means has a
length proportional to the wavelength of an optical signal being
propagated in a respective one of said loop-back optical paths
means.
16. The optical multiplexing/demultiplexing device as claimed in
claim 12, wherein each of said loop-back optical paths means has a
length inversely proportional to the wavelength of an optical
signal being propagated in a respective one of said loop-back
optical paths means.
17. The optical multiplexing/demultiplexing device as claimed in
claim 12, wherein at least one input section of said plurality of
input sections has a light source selected from a group consisting
of a wavelength-tunable light source, a multi-wavelength light
source and a fixed-wavelength light source.
18. The optical multiplexing/demultiplexing device as claimed in
claim 17, further comprising an optical modulator between one input
section of said plurality of input sections and said
wavelength-tunable light source.
19. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein at least one of said signal processing means is
selected from a group consisting of an optical switch means, an
optical merging/splitting circuit means, optical filter means and
an optical coupler means.
20. The optical multiplexing/demultiplexing device as claimed in
claim 19, wherein said optical switch means comprises an optical
matrix switch disposed to straddle a plurality of said plurality of
loop-back optical paths means, optically connecting an individual
one of said plurality of loop-back optical paths means.
21. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein at least one of said signal processing means is
optical node means for transmitting, amplifying, splitting and
merging optical signals.
22. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein at least one of said signal processing means
includes photodetector means and a light source.
23. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein said signal processing means includes signal delay
means.
24. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein said signal processing means includes signal
amplifier means.
25. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein said signal processing means includes wavelength
conversion means.
26. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein said signal processing means includes an optical
bistable device.
27. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein said signal processing means includes optical
pulse regenerating device means.
28. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein said signal processing means includes optical
pulse equalizing means.
29. The optical multiplexing/demultiplexing device as claimed in
claim 2, wherein said signal processing means includes
wavelength-selective optical filter means.
30. The optical multiplexing/demultiplexing device as claimed in
claim 29, wherein said wavelength-selective optical filter means
comprises an arrayed waveguide grating multi/demultiplexer.
31. The optical multiplexing/demultiplexing device as claimed in
claim 29, wherein said wavelength-selective optical filter means
comprises an interference film.
32. The optical multiplexing/demultiplexing device as claimed in
claim 29, wherein said wavelength-selective optical filter means
comprises a ring resonator.
33. The optical multiplexing/demultiplexing device as claimed in
claim 29, wherein said wavelength-selective optical filter means
comprises a Mach-Zehnder interferometer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical multi/demultiplexer
device with loop-back optical paths having an arrayed waveguide
grating, applicable to optical communication systems and optical
switching systems. The device is simple in construction, and can be
fabricated with high yield.
2. Technical Background
Conventionally, an optical add-drop multiplexer (referred to as
ADM) such as the one shown in FIG. 19 is known as a key device for
use in splitting and inserting wavelength-multiplexed optical
signals. The ADM 1 comprises a demultiplexer 2, a multiplexer 3,
and N lines of optical fibers 4a, 4b, . . . 4n.
In the optical ADM 1 circuit, multiplexed input optical signals
consisting of wavelengths .lambda.1, .lambda.2, . . . , .lambda.n
are separated into optical signals of N wavelengths from which
desired optical signals, for example, .lambda.i and .lambda.j, are
outputted. The remaining optical signals are transmitted through
the optical fibers 4a, 4b, . . . 4n, which are multiplexed with the
external signals .lambda.i, .lambda.j, and are outputted as
multiplexed optical signals .lambda.1, .lambda.2, . . . ,
.lambda.n.
Another conventional ADM is shown in FIG. 20.
This ADM 5 is disposed between two optical transmission lines 6, 7,
and comprises a demultiplexer 11, a multiplexer 12, 7 lines of
optical fibers 13a, 13b, . . . 13g and a signal processing device
14 provided for each of the optical fibers 13a to 13g. In this
case, seven wavelengths are shown for brevity, although in general,
any number of wavelengths can be multiplexed.
In the above ADM 5, the multiplexed input optical signal of
wavelengths .lambda.1, .lambda.2, . . . , .lambda.7 is first
separated into optical signals of seven wavelengths by the
demultiplexer 11, and then these optical signals are transmitted by
the corresponding optical fibers 13a to 13g. The separated optical
signals are processed by each of the signal processing device 14,
are converted into electrical signals and are outputted from the
ADM 5 to transmit the information forward. The response to the
forwarded information or to a new piece of information is converted
into an optical signal by the same signal processing device 14, and
is inputted into a corresponding optical fiber 13. The optical
signals transmitted through the optical fibers 13a to 13g are
multiplexed by the multiplexer 12, and are outputted as multiplexed
optical signals of wavelengths .lambda.1, .lambda.2, . . . ,
.lambda.7, and are forwarded to the optical line 7.
Further in this ADM 5, signal processing is carried out on all the
wavelengths, but in general it is irregular to process all the
signals. In such a case, for the wavelengths which need not be
processed, only the optical fibers 13 are needed, and signal
processing devices 14 can be omitted.
Also, there is known an optical delay line memory which delays
pulsed optical signals and stores delayed optical pulses.
The optical delay line memory is classified into two large
categories depending on the operational mode, into a tap type,
represented typically by a parallel distribution type; and a loop
type represented typically by a looping delay type.
FIG. 21 schematically illustrate the parallel distribution type
optical delay line memory.
This optical delay line memory 21 comprises: a fixed wavelength
light source 22; a 1.times.N optical coupler 23 which divides the
optical pulses from the light source 22 into N optical paths; a
plurality of delay fibers 24a, 24b, . . . , 24n which provide delay
times i.tau. (i=1, 2, . . . , N); an N.times.1 optical switch 25
which selects one pulse of the delayed optical pulses given a delay
of i.tau.; and an optical detector 26 which converts the optical
pulses into electrical signals.
This optical delay line memory 21 has an advantage that the
variations in the optical losses in a plurality of transmission
lines are low.
FIG. 22, is a schematic illustration of the looping type delay line
memory.
The optical delay line memory 31 comprises: a fixed wavelength
light source 22; a 2.times.2 optical coupler 32; a delay line
optical fiber 33 which constitutes a loop for propagating the
signal; an optical amplifier 34; an optical switch 35; and an
optical detector 26.
In the above optical delay line memory 31, the optical pulses
forwarded from the fixed wavelength light source 22 are inputted
into the loop containing the delay line optical fiber 33 through a
2.times.2 optical coupler 32. In this loop, when a pulse signal
loops around i times around the loop, the delay time is given by
i.tau. (where i=1, 2, . . . , N). The optical pulses having been
delayed by the desired time duration, pass through the optical
switch 35 by the gating action of the optical switch 35, and are
converted into electrical signals by the optical detector 26. In
this case, the intensity of the input optical pulses to the
2.times.2 optical coupler 32 decreases in principle by 1/4 every
time the pulse loops through the coupler 32; therefore, when the
pluses loop around N times, the intensity decreases to
1/2.sup.(N+1). An optical amplifier 34 is used to compensate for
the loss in intensity.
The advantage of the optical delay line memory 31 is that the scale
of the hardwares for propagating the signal around the loop is
small.
In the meantime, an optical multi/demultiplexer having an arrayed
waveguide grating type, shown in FIG. 23 has been proposed.
This optical multi/demultiplexer (referred to as a
multi/demultiplexer hereinbelow) 41 is provided with N input
waveguides 43, slab waveguides 44, 45 of depressed surface type,
arrayed waveguide grating 46 and N lines of output waveguides 47,
all of which are mounted on a substrate 42. Multiplexed input
signals, of wavelengths constituted by .lambda.1, .lambda.2, . . .
, .lambda.n, inputted into the input waveguide 43 are separated
into N signals of wavelength .lambda.i and output them from the
corresponding output waveguides 47j (j=a, b, . . . , n).
In the above ADM 1, both a demultiplexer 2 and a multiplexer 3 are
used as a pair, therefore, it is necessary to precisely match the
device characteristics of the demultiplexer 2 and the multiplexer
3. However, in practice, it is extremely difficult to manufacture
such identically-matched devices, and their yield has been very
poor. This was one of the reasons for a high cost of such optical
devices.
In the other type of ADM 5 also, as in the above-mentioned ADM 1,
it is necessary that the operating characteristics of the
demultiplexer 11 and the multiplexer 12 be matched precisely.
Therefore, such a system has a disadvantage that a paired device
must be selected carefully from a production lot, thus leading to
low production yield. The configuration of the ADM 5 also has a
problem that it tended to be too large.
Also, because the above mentioned optical delay line memory 21 uses
a 1.times.N optical coupler 23 and an N.times.1 optical switch 25,
it is mandatory to have optical couplers and optical switches of
uniform optical intensity loss and optical division ratio, thus
leading to one major disadvantage that the number of the operating
component parts required increases, and the number of steps in the
joining operation increases. It follows, therefore, that the number
of optical parts for making the system also increases, and the
economics of the system suffers.
Further, as the number of division (N) increases, it becomes
difficult, in particular, to fabricate N.times.1 optical switches
25 for varying the magnitude of optical delay times.
Further, in the above optical delay line memory 31, it is not
possible to make a loop gain of 1, thus leading to the basic
deficiency that the optical intensity loss increases as the optical
pulses are propagated around the loop, and that the spontaneous
emission noise accumulates leading to a degradation in the S/N
ratio.
Further, in the above multi/demultiplexer 41 of the arrayed
waveguide grating type, multiplexed optical signals consisting of
.lambda.1, .lambda.2, . . . , .lambda.n are separated into N pieces
of optical signals .lambda.i, and are outputted from the
corresponding output waveguide 47j. However, there are many unused
input waveguides 43 and output waveguides 47, and the utilization
factor is low, thus wasting the vast multiplexing capabilities of
this optical device.
SUMMARY OF THE INVENTION
The present invention was made in view of the background of the
technology presented above, and the main object is to present an
optical multi/demultiplexer device, of a simple construction and
stable performance, having an arrayed waveguide grating with
loop-back optical paths. Hereinbelow, the optical
multi/demultiplexer device is referred to as the arrayed waveguide
grating multi/demultiplexer (abbreviated as AWGMD) with loop-back
optical paths.
The arrayed waveguide grating multi/demultiplexer with loop-back
optical paths is provided with a common arrayed waveguide grating
shared between a plurality of input sections and a plurality of
output sections. A part of the optical signals from the output
sections is inputted and looped through the corresponding input
section of the plurality of input sections to generate output
optical signals containing optical information. Therefore, the
performance of the optical device of the present invention is
superior to that of using several conventional multiplexers and
demultiplexers of matched performance characteristics.
The optical device of the present invention is an arrayed waveguide
grating multi/demultiplexer for generating optical information from
optical signals inputted into the device, the device comprising:
(a) a plurality of input sections for receiving optical signals
consisting of a plurality of wavelengths, (b) a plurality of output
sections for outputting optical information; (c) a plurality of
loop-back optical paths means disposed between the plurality of
input sections and the plurality of output sections for generating
looping optical signals from the output section, and inputting the
looping optical signals into corresponding ones of the input
section so as to generate optical information.
In the optical device of the above configuration, because
demultiplexing and multiplexing operations are performed by the
same multi/demultiplexer device, the wavelength characteristics of
the demultiplexer and the multiplexer are perfectly matched. The
optical signals are passed through the same devices several times,
and the output signals thus become narrowband. The device
production yield is therefore improved. Further, each wavelength of
the wavelength-multiplexed signals can be processed separately
while minimizing splitting losses, therefore, the optical band
width of the optical signals becomes narrowband, and it becomes
possible to decrease the undesired noise components of the optical
signal spectrum are greatly decreased.
In accordance with the above feature of the optical device of the
present invention, it becomes possible to present an optical device
having low splitting losses, large signal to noise ratio, and a
simplified device construction, thereby leading to significantly
improved and stable production yield.
Optical signal splitting and insertion, delay line memory and delay
equalization functions which are essential in optical information
transmission and switching can all be provided by the same circuit
configuration, thus presenting an optical device which is superior
to the conventional optical devices of similar capability.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of the multi/demultiplexer of the
arrayed waveguide grating type (AWGMD) having optical paths in the
first embodiment.
FIGS. 2(a) to 2(d) are schematic drawings of the examples of the
signal processor in which 2(a) is an optical pulse regenerating
circuit comprising Optical/Electrical converter, Electrical/Optical
converter and waveform reshaping circuit; 2(b) is an optical
amplifier; 2(c) is a signal processor having 2.times.2 switches;
and 2(d) is an optical filter.
FIG. 3 is the second embodiment of the multi/demultiplexer having
the arrayed waveguide grating with loop-back optical paths.
FIG. 4 is an illustration to explain the principle of performing
further division operation on group multiplexed dense
wavelength-division-multiplexed (WDM) optical signals.
FIG. 5 is a schematic drawing of the third embodiment of the AWGMD
with loop-back optical paths.
FIG. 6 is a schematic drawing of the forth embodiment of the AWGMD
with loop-back optical paths.
FIG. 7 is a schematic drawing of the fifth embodiment of the AWGMD
with loop-back optical paths.
FIG. 8 is a schematic drawing showing the signal processor of the
multi/demultiplexer of the fifth embodiment.
FIG. 9 is a schematic drawing of an example of an optical gate
switch as the signal processor in the AWGMD with loop-back optical
paths.
FIG. 10 is a schematic drawing showing the sixth embodiment of the
AWGMD with loop-back optical paths.
FIG. 11 is an illustration to explain the compression of the
grouped time-arranged optical pulse signals.
FIG. 12 is an illustration to explain the division of the grouped
time-arranged optical pulse signals.
FIG. 13 is a schematic drawing showing the seventh embodiment of
the AWGMD with loop-back optical paths.
FIG. 14 is a schematic drawing showing the eighth embodiment of the
AWGMD with loop-back optical paths.
FIG. 15 shows various waveforms of optical pulses.
FIG. 16 is a schematic drawing showing the ninth embodiment of the
AWGMD with loop-back optical paths.
FIG. 17 is a schematic drawing showing the tenth embodiment of the
AWGMD with loop-back optical paths.
FIG. 18 is an illustration to show the state of input/output of the
optical signals in the tenth embodiment.
FIG. 19 is a schematic drawing of the conventional type of optical
add-drop multiplexer (ADM).
FIG. 20 is a schematic drawing of another conventional type of
optical ADM.
FIG. 21 is a schematic drawing of the conventional optical delay
line memory of a parallel distribution type.
FIG. 22 is a schematic drawing of the conventional optical delay
line memory of a circulating loop type.
FIG. 23 is a schematic drawing of the conventional AWGMD.
PREFERRED EMBODIMENTS OF THE INVENTION
The preferred embodiments of the invention will be explained in the
following with reference to the drawings.
First Embodiment
FIG. 1 is a schematic drawing to show the first embodiment of the
arrayed waveguide grating (AWGMD) with loop-back optical paths.
The AWGMD with loop-back optical paths shown in this figure
comprises: optical lines 6, 7; an AWGMD 41 disposed between the
lines 6, 7; output waveguides 47; input waveguides 43; and optical
fibers 51 to loop-back the optical signals outputted from the
output waveguides 47 into the corresponding input waveguides 43;
and respective signal processor 52 in each of the optical fibers
51.
In this embodiment, the optical line 6 is connected to an input
waveguide 43h which is one of the eight input waveguides 43a to
43h; and the optical line 7 is connected to an output waveguide 47h
which is one of the eight output waveguides 47a to 47h.
The seven wavelength-multiplexed optical signals of seven waveforms
having the wavelengths .lambda., .lambda.2, . . . .lambda.7 are
inputted into the input waveguide 43h of the AWGMD 41 after
propagating through the optical line 6. The wavelength-multiplexed
optical signals are diverged by the diffraction effects at the slab
waveguide 44, and are guided into the various waveguides
constituting the arrayed waveguide grating 46.
The optical signals are condensed by the slab waveguide 45 after
propagating in the arrayed waveguide grating 46, but the nodes of
the bundle of lights are different because of the phase differences
generated during the propagation in the arrayed waveguide grating
46. In other words, the various wavelengths .lambda.i are taken out
from the respective waveguides 47j (where J=a, b, . . . , g); for
example, the wavelength .lambda.1 is outputted from the output
waveguide 47a; the wavelength .lambda.2 from the output waveguide
47b, . . . . , and the wavelength .lambda.7 from the waveguide 47g.
The optical signals are thus demultiplexed. The demultiplexed
signals are propagated through the respective optical fibers 51a to
51g, and are guided to the respective signal processors 52a to
52g.
The signal processors 52 receive the optical signals, thereby
obtaining the information transmitted. Each of the signal
processors 52a to 52g is provided with a light source to generate
an optical signal of the same wavelength as the received
wavelength, and the information to be forwarded is superimposed on
the optical signal, and is returned to the AWGMD 41 through the
optical fibers 51.
The optical signal inputted into the input waveguide 43 is
multiplexed by the same effect as that in the first propagation, in
the output waveguide 47h. The important result in this operation is
that the optical fiber 51j of the jth fiber is connected to the
input waveguide 43j of the jth waveguide. The optical signal of the
wavelength .lambda.i inputted from the input waveguide 43j is
outputted from the output waveguide 47h. That is to say that all
the optical signal having the wavelengths .lambda.1, .lambda.2, . .
. , .lambda.7 are forwarded to the optical line 7 from the
waveguide 47h.
In the meantime, the pilot optical signal of wavelength .lambda.0
does not pass through the optical fibers 51 and the signal
processor 52, but is outputted through the input waveguide 43h, the
arrayed waveguide grating 46 and the output waveguide 47h.
As described above, in the arrayed waveguide grating
multi/demultiplexer (AWGMD) of the present invention, it is
possible to perform multiplexing and demultiplexing operations
using one AWGMD 41 by adopting an efficient loop-back
configuration, i.e., looping the demultiplexed optical signal back
to the input side with the use of the optical fibers 51.
In the first embodiment, a signal processor 52 is provided for each
of the fibers 51 so as to be able to process all seven wavelengths.
In general, however, it is not common to provide a signal processor
for all the optical signals of the wavelength-multiplexed optical
signals propagating in the optical line 6. In such a case, there is
no need to provide a signal processor 52 in the optical fibers 51
in which is propagating an optical signal which does not need
processing, and the signal processor 52 should be removed.
Next, the signal processor 52 will be explained in detail with
reference to FIGS. 2(a), 2(b).
The signal processor shown in FIG. 2(a) is an optical pulse
regenerating circuit, and comprises: an optical/electrical (O/E)
converter 53 consisting of a photodiode and its control circuits;
an electrical/optical (E/O) converter 54 consisting of a
semiconductor laser and its control circuits; a waveform reshaping
circuit (not shown), disposed between the O/E converter 53 and the
E/O converter 54, which reshapes the waveform degraded by
propagation. This waveform reshaping circuit has a capability to
regenerate the electrically degraded pulse signals into
rectangular-pulses of the same bit rate.
The optical signals containing the desired information are
converted into electrical signals by the O/E converter 53, and are
outputted from the electrical output terminal 55. The information
to be forwarded is inputted as electrical signals into the
electrical input terminal 56 which are converted into optical
signals by the E/O converter 54, and are outputted into optical
fibers 51 (or into waveguide).
The signal processor shown in FIG. 2(b) comprises a glass waveguide
amplifier; an optical semiconductor amplifier, and an optical
amplifier 57 such as erbium-doped optical fiber amplifier. These
amplifiers regenerate the light intensity of optical signals
degraded by propagating in the optical line 6 and the arrayed
waveguide grating 46.
FIG. 2(c) is an example of using a 2.times.2 optical switch 58 to
connect the signal processor 52 with the optical fibers 51.
The optical switch 58 comprises a 2.times.2 Mach-Zehnder
interferometer made with a silica type glass, an optical
semiconductors, or lithium niobate optical waveguide. The optical
switch 58 has four input/output ports 61 to 64, and when the switch
is in the through-state, port 61 is connected with port 63 and port
62 is connected with port 64, and the optical signals pass through
without being processed. When the ports are cross-connected, port
61 is connected with port 64 and port 62 is connected with port 63,
and the signal processing is performed.
The signal processor shown in FIG. 2(d) includes such devices as
waveguide-type ring resonator having wavelength selectivity;
waveguide-type Mach-Zehnder interferometer; and optical filters 65
using dielectric multilayer film (interference film). When an AWGMD
of higher resolution capability is used, higher precision
multiplexing and demultiplexing of optical signals become possible.
With the use of such AWGMD, optical signal splitting and insertion
of frequency-modulated optical signals become possible.
In the first embodiment, of the many input waveguides 43 and output
waveguides 47, the terminal end input waveguide 43h is connected to
optical line 6, and the terminal end output waveguide 47h is
connected to the optical line 7, but the first embodiment is not
limited only to such a configuration. For example, the same effect
can be obtained by connecting input waveguide 43b with optical line
6, and output waveguide 47b with optical line 7. In general, the
AWGMD 41 provides the highest diffraction efficiency and the lowest
loss when the centrally positioned input waveguides 43 and the
output waveguides 47 are utilized. Therefore, the optical line 6
should be connected to input waveguide 43d (or input waveguide 43e)
which is close to the center, and the optical line 7 to output
waveguide 47d (or output waveguide 47e) near the center.
Further, in this embodiment, the number of wavelength multiplexing
by the AWGMD 41 is chosen to be eight, but the first embodiment is
not limited to this number, and this number can be changed suitably
by changing the design of the arrayed waveguide grating 46.
Further, signal processor 52 can be made of a 2.times.2 optical
coupler. In this case, one of the two optical signals split into
two optical signals by the optical coupler propagates through the
optical fibers 51, and simultaneously, the other signal is
outputted to an external receiver. It is thus possible to monitor
propagated optical signals without severing the optical fibers 51.
It is also possible to insert a new signal of the same wavelength
in the optical fibers 51 with the use of the optical coupler.
Furthermore in the first embodiment, the polarization dependence
can also be eliminated by depositing a layer of amorphous silicon
on or inserting a .lambda./2 plate into the arrayed waveguide
grating 46.
Embodiment 2
FIG. 3 is a schematic illustration of the second embodiment of the
AWGMD with loop-back optical paths. The difference between the
AWGMD with loop-back optical paths in the first and second
embodiment is the provision of an optical fibers 51 between each
output waveguides 47 and the input waveguides 43, and one optical
fiber 51d of the optical fiber bundle, in which an optical signal
having the wavelength .lambda.4 is propagated, is provided with an
AWGMD 41.
In this optical circuit, it is possible to further demultiplex the
group demultiplexed dense wavelength-division multiplexed (dense
WDM) optical signal .lambda.4 (or frequency modulated optical
signal) into wavelengths .lambda.41, .lambda.42, . . . , .lambda.47
(or a plurality of frequency modulated optical signals). It is also
possible to multiplex a plurality of closely spaced wavelengths
.lambda.41, .lambda.42, . . . , .lambda.47 (or a plurality of
frequency-modulated optical signals) into dense WDM optical signals
.lambda.4 (or a frequency modulated optical signals).
According to the above AWGMD with loop-back optical paths, it is
possible to perform further dense multiplexing or demultiplexing on
previously modulated optical signals. It also enables splitting and
inserting of dense WDM signals because of the two-staged splitting
and inserting circuits provided.
Embodiment 3
FIG. 5 is a schematic illustration of the third embodiment of the
AWGMD with loop-back optical paths.
The feature of this AWGMD circuit is that the arrayed waveguide
grating 41, a plurality of waveguides (loop-back optical paths) 71,
and a plurality of signal processors 72 comprising optical
semiconductors are all installed on one common substrate 73. The
operation of and the signal flow of this circuit are the same as
those in the first embodiment.
In this embodiment, because the input waveguides 43, the output
waveguides 47 and the waveguide 71 are disposed on the same
substrate 73, the labor of making connections is eliminated.
Therefore, the number of component parts and the assembly steps are
reduced, thus making the device further compact and increasing the
device reliability.
Further, in this embodiment, the signal processor 72 made of
optical semiconductor waveguides is integrated with the AWGMD 41
made of silica type glass. If the AWGMD 41 were made of optical
semiconductor waveguides, it is possible to fabricate both devices
at the same time on the same substrate 73, thus resulting in
further savings in the manufacturing cost.
Also, although this AWGMD with the loop-back optical paths is
fabricated by having all the devices on a common substrate, it is
also possible to make the circuit by employing laser welding,
optical bonding agents such as light hardening resins and soldering
to bond the various component parts.
Embodiment 4
FIG. 6 is a schematic illustration of the fourth embodiment of the
AWGMD device with loop-back optical paths.
This device comprises: a wavelength-tunable semiconductor laser
source 81; an intensity modulator (optical modulator) 82; a
polarization compensator 83; an optical line 6 on the input side; a
7.times.7 AWGMD 41 made of silica glass; a plurality of delay line
optical fibers (signal delay means) 84; a plurality of signal
processors (optical signal processing means) 85; an optical line 7
on the output side; photodetector element 86.
The wavelength-tunable semiconductor laser source 81 is able to
vary the wavelength of the output laser beam by changing the input
current, for example. In this embodiment, a laser source 81 capable
of generating seven wavelengths, .lambda.1, .lambda.2, . . . ,
.lambda.7 was used.
In the AWGMD 41, it is possible to output an optical signal of a
specific wavelength, for example .lambda.i, generated from the
laser source 81, and output it from the corresponding output
waveguide 47j (j=a, b, . . . , g) of the plurality of waveguides
47.
Also, the delay line optical fibers 84 are provided to correspond
with the respective transmission waveguides between the output
waveguides 47 and the input waveguides 43. For example, the first
output waveguide (first o/w) 47a joins with the first delay line
optical fiber 84a and the first input waveguide (i/w) 43a; second
o/w 47b with second delay line optical fiber 84b and the second i/w
43b; and so on, so that signal light outputted from the output
waveguides 47 will be given a certain delay time.
Next, the operation of the arrayed waveguide grating
multi/demultiplexer (AWGMD) with loop-back optical paths will be
presented.
The optical signal of the wavelength .lambda.i (i=1, 2, . . . , 7)
outputted from the wavelength-tunable laser source 81 is converted
into respective optical signal having respective information by the
signal processing operation of the intensity modulator 82. The
signal is then passed through the polarization compensator 83 to
coincide the polarization plane of the various signals, and, after
passing through the optical line 6 on the input side, is inputted
into the input waveguide 43a to 43g of the AWGMD 41. For example,
the signal pulses inputted into the input waveguide (i/w) 43a are
dispersed by diffraction at the slab waveguide 44, are inputted
into a plurality of waveguides comprising the arrayed waveguide
grating 46, and after passing through the grating 46, are condensed
by the slab waveguide 45.
In this case, the location of interference of diffracted signal,
i.e. the location of condensing light, is determined by the phase
difference generated at the arrayed waveguide grating 46, and this
location is wavelength-dependent. The signal light pulses of
wavelength .lambda.i (i=1, 2, . . . , 7) are outputted from the
corresponding output waveguides 47j (o/w 47j) (j=a, b, . . . , g):
for example, .lambda.1 is outputted from o/w 47a, .lambda.2 from
o/w 47b, . . . , and .lambda.7 from o/w 47g. Of these signal
pulses, all excepting the pulse having the wavelength .lambda.0,
are returned to the input side and are inputted into the i/w 43j
(j=a, b, . . . , g) after having been given a time delay of i.tau.
(i=1, 2, . . . , 7) by passing through the delay line optical
fibers 84j (j=a, b, . . . , 7).
At this time, each of the optical signal pulses given delay times
of i.tau. (i=1, 2, . . . , 7) is outputted simultaneously with the
optical signal pulse that has not been delayed to a common output
waveguide 47h. The optical signal pulses pass through the optical
line 7 on the output side, and are converted into electrical
signals by the photodetector element 86, and thus constitute
delayed information. In other words, the optical signal pulses of
wavelengths .lambda.i inputted into the i/w 43j are forwarded to
delay line fibers 84j via o/w 43j. In this case, if the wavelengths
of the input optical signal pulses are changed to .lambda.1,
.lambda.2, . . . , .lambda.7, the demultiplexed optical signal
pulses can choose o/w 47j (j=a, b, . . . , g), and the delay times
of .tau., 2.tau., . . . , 7.tau. in accordance with the delay line
fibers 84.
As explained above, the AWGMD with loop-back optical paths
comprises a wavelength-tunable semiconductor laser source 81; an
intensity modulator (optical modulator) 82; a polarization
compensator 83; an optical line 6 on the input side; a 7.times.7
AWGMD 41 made of silica glass; a plurality of delay line fibers
(signal delay means) 84; a plurality of signal processors (optical
signal processing means) 85; and an optical line 7 on the output
side; a photodetector element 86. Therefore, it is possible to
change the time delay of optical pulses freely and quickly.
Also, because the wavelength switching are performed using a
wavelength-tunable semiconductor laser source 81, switching to
selected wavelength can be performed readily. Also, because the
variable delay times are produced by one AWGMD 41, it is possible
to minimize the variations due to variables associated with a
number of devices. The yield of the circuit is thus increased.
It is also possible to prevent an increase in the splitting loss
with the increase in delay times.
Further, the light signal pulses are passed through the AWGMD 41
twice, through the delay line fibers 84, the bandwidth of the
signal pulses becomes a narrowband, therefore, it becomes possible
to significantly decrease the noise component of the optical signal
spectrum.
Further in this embodiment, although a wavelength-tunable
semiconductor laser source 81 was used as the variable wavelength
light source, it is not limited to this device, and other light
sources can be used. For example, distributed Bragg reflected (DBR)
multi-electrode semiconductor laser, distributed feedback (DFB)
semiconductor laser, Farby-Perot (FP) semiconductor laser, external
cavity semiconductor laser may also be used to produce the same
effect as the wavelength-tunable laser source 81 used in this
embodiment.
Further for the wavelength-tunable laser source, it is also
permissible to use a combination of N laser sources having a fixed
but differing wavelengths with an N.times.1 optical coupler, a
combination of N laser sources having a fixed but differing
wavelengths with an optical gate switch and multiplexing with an
N.times.1 optical coupler. In the former optical circuit, the
wavelength can be changed by switching the N.times.1 optical
coupler, and in the latter circuit, by turning on the optical gate
switches.
Further, by changing the lengths of the delay line fibers, it is
possible to change the duration and the range of the time
delay.
Embodiment 5
FIG. 7 is an illustration of the fifth embodiment the arrayed
waveguide grating multi/demultiplexer (AWGMD) with loop-back
optical paths.
The difference in the AWGMD devices between the fourth and the
fifth embodiments is that the substrate 91 has the following
devices integrally fabricated thereon; i.e. a wavelength-tunable
semiconductor laser source 81 integrated with an intensity
modulator 82 serving a wavelength-tunable optical transmitter 92
(wavelength-tunable light source, optical modulator); lensed fibers
93, an AWGMD 41; delay line waveguides (optical delay means) 94
replacing the delay line fibers 84; a plurality of optical signal
processors 85.
In this embodiment, the two devices are connected with lensed
fibers 93, because of the size difference between the optical
transmitter 92 and the AWGMD 41, since it is difficult to connect
them directly.
Further, the following fabricated devices may be used for the
optical signal processor 85.
In FIG. 8, optical signal processor 85 is made by incorporating an
optical amplifier 95 such as semiconductor amplifier or glass
waveguide amplifier in the delay line waveguide 94, and this
optical amplifier 95 compensates for the loss in signal intensity
generated in the transmission paths and in the AWGMD 41.
In FIG. 9, optical signal processor 85 is made by incorporating an
optical gate switch 96, such as lithium niobate (LiNbO.sub.3)
optical modulator or a semiconductor switch in the delay line
waveguide 94. These devices perform optical signal processing by
passing or blocking a part of optical signal by turning on the
optical gate switch 96 or the wavelength-tunable optical
transmitter 92.
As explained above, the AWGMD of the present embodiment comprises
an integrated circuit on a common substrate 91 including such
devices as wavelength-tunable optical transmitter 92; lensed fiber
93; AWGMD 41; delay line waveguide 94; optical processors 85.
Therefore, the circuit is able to provide the same functions as the
AWGMD with loop-back optical paths presented in the embodiment 3.
Also, the connections and the connecting steps required for
connecting the laser source 81 with the intensity modulator 82 can
be eliminated, and the polarization compensator 83 between the
intensity modulator 82 and the AWGMD 41 can be eliminated.
Therefore, the circuit can be made even more compact, and the
number of parts required and the fabrication steps can be
reduced.
Also, although this integration was made on a common substrate, it
is also possible to make this circuit by employing laser welding,
optical bonding agents such as light hardening resins and soldering
to bond the various component parts.
Also in this embodiment, the wavelength-tunable optical transmitter
92 and the AWGMD 41 was connected with lensed fibers 93, other
optical merging techniques can be utilized. For example,
guided-wave spot size converter to effectively connect the two
devices can be fabricated on the same substrate as the AWGMD 41
thereby further making the circuit more compact.
Embodiment 6
FIG. 10 is an illustration of the sixth embodiment of the AWGMD
with loop-back optical paths.
The differences in the AWGMD between this embodiment and the fifth
embodiment are that, the lengths of the plurality of delay line
waveguides (optical delay means) 97, which joins the input
waveguides 43 and the output waveguides 47 on the same substrate
91, are made to be inversely proportional to the respective
propagating wavelengths; and that the optical line 6 is connected
to an external laser source.
In this circuit, the delay line waveguides 97 are made so that the
length becomes longer the shorter the wavelength being propagated
therein, therefore, it is possible to compress or separate the
time-sequenced optical pulses arranged in the wavelength order on
the time axis, or to arrange the optical pulses in the wavelength
order, at a same time point, and to control their positions on the
time axis. For example, optical pulse groups on the time axis can
be compressed or separated.
The compression of an optical pulse group on the time axis will be
explained in more detail.
In general, when the optical pulses propagate in optical fibers,
the pulse width of the optical pulse tends to widen as a result of
mode dispersion during the pulse transmission, or chirping in the
semiconductor laser source. In this case, it is assumed that the
wavelength components of the widened pulse are the same as those of
the pulses multiplexed on the time axis. In other words, as shown
in FIG. 11, the pulses for inputting are those short wavelength
pulse group, .lambda.N, . . . , .lambda.2, .lambda.1, which
propagate faster.
In the circuit of this embodiment, the lengths of the plurality of
the delay line waveguides 97, through which the demultiplexed
optical pulses pass, are adjusted so that the time interval T
between the pulses is the same as the delay time .tau. between the
neighboring delay line waveguides so that the long wavelength
components propagate faster than the short wavelength components.
The result is that the pulses having short wavelength components
are delayed, and the expanded pulses on the time axis are
compressed on a plurality of delay line waveguides 97 having the
reverse delay properties.
Next, the separation of the time-sequenced pulse group will be
explained.
For example, a multi-wavelength generating semiconductor laser beam
can be regarded as a synthesized beam comprising a number of
simultaneously generated wavelengths, .lambda.1, .lambda.2, . . . ,
and .lambda.N. When this laser beam is externally multiplexed to
produce simultaneously generated wavelengths .lambda.1, .lambda.2,
. . . , .lambda.N, the short wavelength components propagate slower
than the long wavelength components. Therefore, when the pulse
group passes through the plurality of delay line waveguides, the
component pulses distribute themselves on the time axis, as shown
in FIG. 12. Thus, the simultaneously generated optical pulse group,
comprising .lambda.N, . . . , .lambda.2, .lambda.1, can be
separated on the time axis.
As explained above, the AWGMD with loop-back optical paths is
fabricated so that the lengths of the plurality of delay line
waveguides 97 joining the input waveguides 43 and the output
waveguides 47, whose circuits are formed on the same substrate 94,
are inversely proportional to the wavelengths being propagated
therein. Therefore, the AWGMD of this embodiment enables to
compress or separate the optical pulse group comprising pulses
arranged in the order of wavelengths on the time axis.
Embodiment 7
FIG. 13 is an illustration of the seventh embodiment of the AWGMD
with loop-back optical paths.
The difference between the AWGMD with loop-back optical paths of
this embodiment and that in the first embodiment is that one signal
processor 52b is served by a wavelength converter 101, and the slab
waveguide 45 is provided with new output waveguides 47K, 47m, which
are connected with new optical transmission lines 102k, 102m.
The wavelength converter 101 comprises: an O/E converter 103 to
convert optical signal to electrical signal; and an E/O converter
104 which activates another laser source having another wavelength
based on the electrical signal. However, other configurations are
possible, for example converters utilizing nonlinear crystals such
as potassium titanium phosphate (KTP), lithium niobate
(LiNbO.sub.3) lithium tantanate (LiTaO.sub.3), or acousto-optic
modulators (A/O modulator) based on crystalline materials such as
molybdenum plumbate (PbMbO.sub.4), tellurium dioxide
(TeO.sub.2).
In the AWGMD with loop-back optical paths, of the plurality of
multiplexed optical signals propagated in the optical line 6 and
inputted into input waveguide 43h, the optical signal converted by
the wavelength converter 101 is not outputted from the optical line
7, but is outputted from another optical fiber 102 after
propagating through the output waveguide 47. For example, if an
optical signal demultiplexed into the optical fibers 51 having the
wavelength .lambda.2 is converted to a wavelength .lambda.3, an
optical signal having the wavelength .lambda.3 can be forwarded to
the optical fiber 102k through the output waveguide 47. Therefore,
if the AWGMD with loop-back optical paths of the seventh embodiment
is used as nodes in a ring network, it would be possible to exit
the ring network and select an optical route to propagate to an
external node or a terminal station.
Embodiment 8
FIG. 14 illustrates the eighth embodiment of the AWGMD with
loop-back optical paths.
The difference between the eighth embodiment and the first
embodiment is that one of the signal processor 52b includes an
optical bistable device 111.
The optical bistable device 111 is an application of the non-linear
optic effect of semiconductor lasers having a saturable absorption
region. As shown in FIG. 15, when an optical pulse is inputted into
an optical bistable device 111, the device 111 enters an
oscillating state because of the non-linear optic effect. When an
electrical reset pulse is impressed on the saturable absorption
region, the device 111 changes to a non-oscillating state.
Therefore, it enables to generate a new optical signal whose
ON-period is between the input of a trigger optical pulse and the
impression of an electrical reset signal. It is clear that the
duration of the ON-period can be altered suitably.
Embodiment 9
FIG. 16 is an illustration of the ninth embodiment of the AWGMD
with loop-back optical paths.
The difference between the ninth embodiment and the first
embodiment is that one slab waveguide 44 is provided with the end
terminals of the arrayed waveguide grating 46, the slab waveguide
44 is provided with the input waveguide 43 and the output waveguide
47, and the signal processors 85 are removed from the delay line
waveguides.
In this circuit, the lengths of each of the waveguides of the delay
line waveguides are chosen to be inversely related to the short
wavelength components, .lambda.s. Therefore, the delay line
waveguides 97 function as the normal dispersion medium having a
larger dispersion coefficient than that of the zero-dispersion
wavelength of the optical fiber having a dispersion shift
wavelength of 1.3 .mu.m.
For example, when an optical pulse having a wavelength of 1.55
.mu.m propagates through an optical fiber having a 1.3 .mu.m
dispersion shift, the optical fiber functions as an anomalous
dispersion medium, thus causing the shorter wavelength components,
.lambda.s, to propagate faster than the longer wavelength
components, .lambda.1. Therefore, it means that the width of the
propagating pulses expands.
When the expanded-width optical pulses are inputted from the
optical line 6 into the AWGMD with loop-back optical paths of this
embodiment, the closer the short wavelength components is to the
leading edge of the optical pluses, the slower their propagation
speeds. Similarly, the closer the long wavelength component is to
the trailing edge of the optical pulses, the faster their
propagation speeds. It follows that the delay line waveguide 97
acts as a normal dispersion medium having reverse dispersion
properties, and is capable of narrowing the pulse width caused by
wavelength dispersion, and in effect performs so-called dispersion
compensation.
On the other hand, if the length of each of the waveguides of the
delay line waveguides 97 is made to be proportional to the
respective wavelength component of the optical signals, the delay
line waveguides 97 can function as an anomalous dispersion medium.
For example, it enables to compensate (equalize) pulse width
broadening of optical pulses having a wavelength shorter than the
zero-dispersion wavelength of a 1.3 .mu.m dispersion shift optical
fiber.
The dispersion compensation function of the delay line waveguides
97 is able to generate the same effect as presented above for
optical pulses of any wavelengths to be transmitted by setting the
delay times to correspond with the dispersion values.
Further, because the circuit uses only one slab waveguide 44, the
entire AWGMD with loop-back optical paths can be made compact.
Embodiment 10
FIG. 17 is an illustration of the tenth embodiment of the AWGMD
with loop-back optical paths.
The difference between the tenth embodiment and the eighth
embodiment is that optical fibers 51a to 51d are provided between
each of the output waveguides 47 and the input waveguides 43, that
each of the optical fibers 51a to 51d is provided with nodes 121a
to 121d, and that a 4.times.4 optical matrix switch 122 is provided
straddling the optical fibers 51a to 51d.
In the tenth embodiment, it is possible to select any optical path
by operating the optical matrix switch 122 to switch the input
waveguides 43a to 43d for returning a plurality of optical signals.
In this embodiment, the waveguide 43e is used as the input terminal
for the wavelength division multiplexing signal, and the output
waveguide 47e is used as the output terminal, there are, in
principle,
ways of selecting the optical paths.
The operation of the AWGMD with loop-back optical paths of the
tenth embodiment will be explained below with reference to FIG.
18.
By operating the optical matrix switch 122, for example, the output
waveguide (o/w) 47a is connected to the input waveguide (i/w) 43b;
o/w 47b to i/w 43a; o/w 47c to i/w 43d; o/w 47d to i/w 43c, then an
optical signal having the wavelength .lambda.1 is outputted to
optical transmission line 7 through the nodes 12b, 121c, 121d and
121a. Similarly, an optical signal having the wavelength .lambda.2
passes through the nodes, 121a and 121b; an optical signal having
the wavelength .lambda.3 passes through the nodes, 121d, and 121c;
an optical signal having the wavelength .lambda.4 passes through
the nodes 121c, 121b, 121a and 121d to be transmitted to optical
line 7.
In this embodiment, each optical signal is outputted to the common
optical line 7 after passing through more than one node 121a to
121d provided on the optical fiber bundle 51.
On the other hand, the pilot signal of .lambda.0 does not pass
through the optical matrix switch 122 and the nodes 121a to 121d,
but it passes through input waveguides 43e, arrayed waveguide
grating 46 and the output waveguide 47e, and is outputted to
optical line 7.
As described above, by switching the optical matrix switch 122, it
is possible to suitably switch the connections between the output
waveguides 47a to 47d and the input waveguides 43a to 43d, and to
select the nodes, 121a to 121d to be passed through. Further, even
when using only one wavelength, by setting the optical path by
switching the optical matrix switch 122, it is possible to select
more than one suitable nodes of the nodes 121a to 121d to pass
through. The order of passing through the nodes of the nodes 121 to
121d can also be suitably selected. Further, by operating the
optical matrix switch 122 at high speed, the optical paths for
passing the optical pulses shift with time, therefore, the optical
matrix switch 122 can serve as a temporary optical memory for
storing the optical cells or optical packets, which are groups of
optical pulses, for certain specific interval of time.
* * * * *